Portion of an attachment arm



FIG. 1 is a top plan view of a portion of an attachment arm showing our new design;

FIG. 2 is a rear view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is left side view thereof;

FIG. 5 is right side view thereof; and,

FIG. 6 is a top perspective view thereof.

The broken line showing of the bottom of the arm and of the opening in the front side is for illustrative purposes only and forms no part of the claimed design. The unshaded regions including the bottom and bottom edge of the arm form no part of the claimed design. 

The ornamental design for a portion of an attachment arm, as shown and described. 